Galaxy holds stray planets

by Thomas H. Maugh II - May. 22, 2011 12:00 AMLos Angeles Times

The Milky Way galaxy may be filled with millions upon millions of Jupiter-sized planets that have escaped their solar systems and are wandering freely in space, researchers say in a finding that could make astronomers rethink their ideas about planetary formation.

While scientists had previously thought that about 20 percent of stars had massive planets attached to them, the new results, reported in the journal Nature last week, suggest that there are at least twice as many planets as stars - perhaps several times as many.

The finding "is a revelation in the sense that it looks like a quintupling of the number of gas giants in the universe," said astronomer Alan Boss of the Carnegie Institution of Washington, who was not involved in the research.

The discoverers of the wandering planets speculate that the orphan bodies were ejected from formative solar systems soon after they condensed from the interstellar dust that also formed the stars.

"If they are ejected, it's a real puzzle as to how that happened, because you have to be kicked out by something bigger than you," Boss said.

The finding will have "theorists scratching their heads and sharpening their pencils for some time to come," he added.

The orphan planets were discovered by an international team of astronomers headed by Takahiro Sumi of Osaka University in Japan. The team used the 5.9-foot telescope at Mount John University Observatory in New Zealand to regularly scan the innumerable stars at the center of the galaxy to search for so-called gravitational-microlensing events.

When a massive object passes between Earth and a distant star, the gravity of that object bends the light waves around it, acting like a magnifying lens to make the star appear brighter. How long the object remains brighter is a measure of the mass of the intervening body. Stars can cause the distant object to appear brighter for weeks, while Jupiter-sized objects produce a similar effect for days.

The Japanese team reported that they observed 10 Jupiter-sized objects, each at a distance of about 10,000 to 20,000 light-years from Earth. Close examination of the data showed that there were no stars within a billion miles or so of the planets, about the distance from our sun to the outer solar system. The team therefore concluded that the objects were in orbits much larger than any previously observed or that they are wandering freely through interstellar space.